1. Field of the Invention
The present invention relates to a resonant switching power supply device and particularly to a resonant switching power supply device able to reduce an increase in switching frequency when a load is light.
2. Description of the Related Art
As a stabilized direct-current power supply device, a resonant switching power supply device has been used. The resonant switching power supply device is equipped with: high-side and low-side switching elements that are connected in series between the cathode and anode of an input direct-current power source; and a resonant circuit that is connected in parallel to one of the switching elements and consists of a primary winding of a transformer and a current resonant capacitor. A resonant current flows through a primary-side inductance element of the transformer and the current resonant capacitor as the switching elements are turned on and off. At this time, the voltage obtained at a second winding of the transformer is rectified. A direct-current voltage is therefore obtained at the output side.
The resonant switching power supply device can adopt a control method called PFM (Pulse Frequency Modulation) control to control an output voltage by changing the switching frequency. The PFM control makes use of the following characteristic: the output voltage rises when the switching frequency is set close to a resonant frequency of the primary-side inductance element of the transformer and the current resonant capacitor, and the output voltage decreases when the switching frequency is set away from the resonant frequency. When a higher frequency range than the resonant frequency is used as the switching frequency, the switching frequency is raised while the output voltage is decreased, and the switching frequency is lowered while the output voltage is increased. In this case, the output voltage is fed back and the switching frequency of the switching elements is automatically controlled so that an output direct-current voltage is stabilized.
In such a resonant switching power supply device, a switching operation causes noises, which have a bad effect on other devices. Therefore, various steps are taken to shield the devices from the noises.
For example, what is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 51-144911 (Patent Document 1) is a technique of disposing an electrostatic shield plate inside and outside a primary winding of a transformer in such a way that the primary winding of the transformer is covered with; similarly disposing an electrostatic shield plate in such a way that a secondary winding of the transformer is covered with; connecting the electrostatic shield plate that covers the primary winding of the transformer to the positive side of a primary-side power source; and connecting the electrostatic shield plate that covers the secondary winding of the transformer to GND of a secondary-side power source. Therefore, a noise current resulting from a stray capacitance of the transformer's primary winding is sent back to the primary-side power source through the electrostatic shield plate; a noise current resulting from a stray capacitance of the transformer's secondary winding is grounded to the secondary GND through the electrostatic shield plate. A common mode noise is therefore reduced.
What is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 54-7531 (Patent Document 2) is a technique of providing an electrostatic shield plate between the primary and secondary windings of a transformer; and connecting the electrostatic shield plate to one end of the transformer's primary winding. In this case, the stray capacitance is divided into two capacitors, one between the primary winding and the electrostatic shield plate and the other between the electrostatic shield plate and the secondary winding; the two capacitors are connected in series. Accordingly, in terms of an equivalent circuit, the capacitance becomes smaller and the noise is therefore reduced. Moreover, in the half-bridge power source, a flat loop extending from the stray capacitance to the electrostatic shield plate and the primary winding is formed after the electrostatic shield plate is connected to the middle point of the capacitor. Therefore, a noise current resulting from the stray capacitance of the primary winding is sent back to the primary winding of the transformer.
What is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 60-174064 (Patent Document 3) is a technique of providing a two-layer electrostatic shield plate between the primary and secondary windings of a transformer; connecting to the positive or negative side of an input power source the electrostatic shield plate that is closer to the primary winding; and connecting to the positive or negative side of a secondary-side output the electrostatic shield plate that is closer to the secondary winding. The technique enables a common mode noise to be reduced.
What is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 58-22579 (Patent Document 4) is a technique of inserting a ring-shaped core into an input/output line of a transformer. The technique enables an output terminal noise, an input terminal noise and a radiation noise to be reduced.